Do you see the free space in the LUN listing? After running the space reclaim, the free space seen by ONTAP (for the LUN) should closely match the free space seen by Windows. The free space at the volume level in ONTAP may not match initially because there are probably snapshots of the LUN from before the space reclaim (especially if the snapshot reserve is set to 0%). Eventually, when those snapshots get old and are deleted, you will see a decrease in reported usage at the volume level.
For example, here is the LUN free space before and after a reclaim:
After speaking with support it turns out the problem was my lack of knowledge on how a LUN works. Before this, everything I've done has been with NFS volumes. What I learned was that a LUN will never shrink in size. Once it's grown, there's no reclaiming that space. The dangers in that is making sure you now have enough space in your volume to maintain your snapshot retention policy.
Shrinking the NTFS volume inside the LUN, however, worked perfectly.
Re: Invoke-NAHostVolumeSpaceReclaim not showing free space on filer